The prior art is relatively extensive as regards MIMO systems.
In such transmission systems implementing a plurality of sending and/or receiving antennas, it is classically sought to exploit spatial diversity to the utmost in order to increase the flow rate or the efficiency of transmission, or again to obtain a compromise between these two constituent features.
However, one drawback of the techniques enabling efficient exploitation of spatial diversity, such as space-time block codes (STBC), for example of the Alamouti type, is that they do not make it possible to optimize transmission capacity. In other words, these prior-art techniques can be used only for data transmission at low bit rates.
Conversely, multiple-antenna transmission techniques, which provide for optimal efficiency, as in the case for example of techniques based on spatial multiplexing, have the drawbacks of requiring at least as many antennas when receiving as when sending, not making optimal use of diversity and increasing complexity, mainly at reception.
Other techniques based on linear dispersion codes have then been proposed. These techniques seek a compromise between STBC codes and spatial multiplexing by optimizing both capacity and exploitation of diversity in conjunction. Unfortunately, no method for the systematic building of such optimal codes has been proposed to date.
It is also possible to cite pre-encoding techniques based on the use of unitary encoding matrices. Although these techniques enable optimal use of diversity, they rely on the use of large-sized matrices, which are not always compatible with the use of small-sized transmission frames.
Combined with spatial multiplexing, these linear pre-encoding techniques make it possible, in the case of certain antenna configurations, to optimize the diversity and capacity of a transmission system. However, these optimal performance values are attained solely at high signal-to-noise ratios and for antenna configurations where the number of transmitting antennas is smaller than or equal to the number of receiving antennas.
Another major drawback of these systems lies in the receiver used, which must be of a maximum likelihood type, also known as ML type. These ML receivers are complex to implement and, because of their complexity, they restrict the size of the pre-encoding matrix to the number of transmitting antennas of the system.
There is therefore a need for a novel technique to increase spectral efficiency and the performance of a radio transmission while at the same time achieving transmission of very low complexity, especially at reception.